The present invention relates generally to gas monitoring and control systems and more particularly to devices useful in quantitatively detecting narcotic gases or vapors within systems for administration of anesthetic gases to surgical patients.
Present anesthesia techniques provide for administration of controlled amounts of oxygen and one or more narcotic gases such as nitrous oxide or halogenated hydrocarbons. Halogenated materials commonly employed include, e.g., 1-bromo-1-chloro-2-2-2-trifluoroethane; 1,1,1-trifluoro-2-chloro-2-bromoethane (halothane); 1-bromo-1-chloro-2,2-difluoropropane; 2,2-dichloro-1,1-difluoroethylmethyl-ether. Administration of these materials in gaseous form is attended by numerous difficulties, including the delivery of exact quantities of anesthetic for inhalation by the patient, disposal of excess quantities exhaled by the patient, and/or recovery and re-use of exhaled gas. It is noteworthy, for example, that more precise knowledge of the quantities of anesthetic gases actually taken up by a patient during a surgical procedure can provide useful information concerning the physiological state of the patient (e.g., increases or decreases in cardiac output) as well as the structural integrity of the delivery system. Integrity of delivery systems and maintenance of a pollution-free atmosphere in operating rooms has become a more prominent concern as a result of recent evidence of possible carcinogenic activity of some anesthetic gases.
Proposed solutions to the problem of direct quantification of anesthetic gas within delivery systems have included both electromechanical and electromechanical-optical apparatus. U.S. Pat. No. 3,465,753, for example, relates to gas analysers wherein anesthetic concentration in a given volume of fluid is detected through fluorometric techniques. U.S. Pat. Nos. 3,498,309 and 3,536,088 relate to devices wherein length fluctuations generated by reversible swelling of elastic (e.g., silicone rubber) strips under constant tension and in contact with the narcotic gas delivered to a patient is translated into motion of a mechanical indicating member or actuation of optical means for generating an electrical signal.
While the above-mentioned prior art systems purport to accurately and reproduceably provide quantitative information, their use has revealed substantial functional inadequacies.
Fluorometric systems such as disclosed in the U.S. Pat. No. 3,465,753, for example, are notoriously inaccurate due to the inherent inadequacy of gas sampling schemes wherein only a small quantity of the gas to be analyzed is sampled. Further, fluorometric analysis ordinarily employs rather expensive and intricate light generating and sensing apparatus which provides temporally discontinuous, and therefore marginally useful, information concerning gas concentration.
Devices employing elastic strips as in the U.S. Pat. No. 3,498,309 and 3,536,088 are known to be especially susceptible to variations in temperature, water vapor content of the gas sampled, and mechanical vibration brought about by pressure changes in gas flow. In practice, their use has been essentially limited to monitoring input of anesthetic gas in gaseous flow delivered to a patient rather than anesthetic gas exhaled by the patient. Information so generated provides the anesthesiologist with substantially less than complete knowledge of the ongoing dynamics of the anesthesization process. When nitrous oxide and halothane mixtures are employed, for example, knowledge of halothane input concentration alone provides no information concerning the dynamic operation of "the second gas effect" upon the patient.
Other disadvantages of prior art apparatus measuring length variations in elastic strips include the difficulty in re-use, repair and replacement of components. It is noteworthy, for example, that while the devices of the U.S. Pat. Nos. 3,498,309 and 3,536,088 admit replacement of the silicone rubber strip elements, difficult manipulations are involved owing to the need to releasably secure both ends of the multiple strips with hooks or other similar fixing means. Because gas quantities are ascertained as a function of strip length changes, great care must be exercised in securing proper tension on the strips. Length-change-sensing means provided on the devices also dictate an interrelationship of elements rendering sterilization of the entire device prior to re-use impossible.
There has developed, therefore, a substantial need for gas anesthetic monitoring devices of relatively simple construction which would provide for generation of temporally continuous information concerning precise quantities of anesthetic gas in either patient input or output portions of the system. Ideal devices would be minimally sensitive to fluctuations in temperature, pressure and water vapor concentration, would be easily sterilizable and would incorporate component parts that are easily replaceable.